Photodetector circuit and semiconductor device

ABSTRACT

To provide a photodetector circuit capable of obtaining signals in different periods without being affected by characteristics of a photoelectric conversion element. The photodetector circuit has n signal output circuits (n is a natural number of 2 or more) connected to the photoelectric conversion element. Further, the n signal output circuits each include the following: a transistor whose gate potential varies in accordance with the amount of light entering the photoelectric conversion element; a first switching element which holds the gate potential of the transistor; and a second switching element which controls a signal output from the transistor. Thus, after data based on the amount of light entering the photoelectric conversion elements is held as the gate potentials of the transistors, the second switching elements are turned on, whereby signals in different periods can be obtained without being affected by characteristics of the photoelectric conversion element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photodetector circuit and asemiconductor device including the photodetector circuit.

2. Description of the Related Art

In a variety of fields, semiconductor devices including circuits(hereinafter, also referred to as “photodetector circuits”) whichreceive light from the outside and output signals corresponding to theamount of incident light are used.

Examples of photodetector circuits are a photodetector circuit includinga CMOS circuit (hereinafter, also referred to as a CMOS sensor), and aCMOS sensor includes a photoelectric conversion element (e.g., aphotodiode) which enables current corresponding to the amount ofincident light to flow and a signal output circuit which holds apotential based on the amount of light entering the photoelectricconversion element and outputs a signal corresponding to the potential.

Note that a CMOS sensor detects the amount of light entering aphotoelectric conversion element by performing, in a signal outputcircuit including a MOS transistor, an operation in which a potential(also referred to as charge) held in the signal output circuit isinitialized (also referred to as a reset operation), an operation inwhich a potential corresponding to the amount of photocurrent flowingthrough the photoelectric conversion element is generated (also referredto as a potential generation operation), and an operation in which asignal corresponding to the potential is output (also referred to as anoutput operation).

As an example of semiconductor devices including photodetector circuits,an image display device in which a photodetector circuit is provided ineach of a plurality of pixels arranged in a matrix can be given (e.g.,see Patent Document 1).

In the image display device, in the case where an object to be detected(e.g., a pen or a finger) exists on a display screen, part of lightemitted from the image display device is reflected by the object to bedetected and the amount of reflected light is detected by thephotodetector circuit, whereby a region on the display screen where theobject to be detected exists can be detected.

Further, as an example of semiconductor devices including photodetectorcircuits, a medical diagnostic imaging device provided with ascintillator and a flat panel detector including a plurality ofphotodetector circuits can be given (e.g., see Patent Document 2).

In the medical diagnostic imaging device, a human body is irradiatedwith radiation (e.g., X-rays) emitted from a radiation source, radiationwhich passes through the human body is converted to light (e.g., visiblelight) by the scintillator, and imaging data is composed by detectingthe light with a photodetector circuit included in the flat paneldetector, whereby an image of the inside of the human body can beobtained as electronic data.

However, in a semiconductor device which obtains a variety of data withthe use of a photodetector circuit provided therein as described above,a signal (also referred to as a detection signal) output from thephotodetector circuit is a composite signal including not only a signalneeded for obtaining data (also referred to as an essential signal) butalso an unnecessary signal (also referred to as a noise signal) in somecases.

For example, in the above image display device, a signal correspondingto “light which is reflected by the object to be detected to enter thephotodetector circuit”, which is output from the photodetector circuit,is an essential signal; on the other hand, a signal corresponding to“light (external light) which enters from the outside of the device,such as sunlight or fluorescent light”, which is output from thephotodetector circuit, is a noise signal.

Further, in the above medical diagnostic imaging device, since in lightemitted by the scintillator, there occurs a phenomenon (what is calledafterglow) in which light emission continues even after radiationemission stops, light received by the flat panel detector might includeboth “light emitted due to radiation emission” and “light emitted byafterglow”.

In this case, a signal corresponding to “light emitted due to radiationemission” which is output from a photodetector circuit is an essentialsignal; on the other hand, a signal corresponding to “light emitted byafterglow” which is output from a photodetector circuit is a noisesignal.

In order to solve the above-described problem in that a detection signaloutput from a photodetector circuit includes not only an essentialsignal but also a noise signal, it is effective to remove only a noisesignal selectively from a composite signal. To achieve that, forexample, as an image display device, a device including photodetectorcircuits (CMOS sensors) arranged in a matrix is proposed as inNon-Patent Document 1.

In an image display device in Non-Patent Document 1 (see FIG. 3 inNon-Patent Document 1), in each of photodetector circuits (referred toas photosensors in Non-Patent Document 1) arranged in a matrix, atransistor M1, a transistor M2, and a capacitor C_(INT) function as asignal output circuit and an element D1 functions as a photoelectricconversion element.

In addition, after a reset operation and a potential generationoperation are performed in the photodetector circuits in odd-numberedrows in a period during which an object to be detected is irradiatedwith light by turning on a backlight, a reset operation and a potentialgeneration operation are performed in the photodetector circuits ineven-numbered rows in a period during which the object to be detected isnot irradiated with light by turning off the backlight.

Note that the time interval of blinking the backlight is short, and itcan be considered that the object to be detected hardly moves betweenwhen the backlight is on and when the backlight is off.

After that, output operations are performed at the same time in thephotodetector circuits in two adjacent rows, and a difference betweendetection signals thereof is obtained. Then, this operation is performedsequentially, so that output operations are performed in thephotodetector circuits in all the rows.

A difference between detection signals thus obtained using photodetectorcircuits in two adjacent rows is an accurate signal including only anessential signal because a signal (noise signal) corresponding to theamount of light entering the photodetector circuit when the backlight isoff is removed from a signal (composite signal) corresponding to theamount of light entering the photodetector circuit when the backlight ison.

In other words, a plurality of detection signals (at least two or moredetection signals) are obtained using photodetector circuits, and anaccurate detection signal is obtained using the plurality of detectionsignals.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-079589-   [Patent Document 2] Japanese Published Patent Application No.    2003-250785

Non-Patent Document

-   [Non-Patent Document 1]K. Tanaka, et al., “A System LCD with Optical    Input Function using Infra-Red Backlight Subtraction Scheme”, SID    2010 Digest, pp. 680-683

SUMMARY OF THE INVENTION

However, in a structure described in Non-Patent Document 1, at least twoadjacent photodetector circuits are needed in order to obtain detectionsignals in different periods (when the backlight is on and when thebacklight is off).

Therefore, in the case where there is a difference betweencharacteristics (e.g., light-receiving sensitivity) of photoelectricconversion elements of two photodetector circuits, a detection signaloutput from the two photodetector circuits include the differencebetween characteristics of the photoelectric conversion elements.

In view of the above problem, an object of one embodiment of thedisclosed invention is to provide a photodetector circuit capable ofobtaining detection signals in different periods without being affectedby characteristics of a photoelectric conversion element.

In addition, an object of one embodiment of the disclosed invention isto provide a semiconductor device including the above photodetectorcircuit.

In order to solve the above problem, in one embodiment of the disclosedinvention, a photodetector circuit has a configuration in which n signaloutput circuits (n is a natural number of 2 or more) are connected toone photoelectric conversion element. Further, the n signal outputcircuits each include the following: a transistor whose gate potentialvaries in accordance with the amount of light entering the photoelectricconversion element and which outputs a signal corresponding to the gatepotential; a first switching element which is connected between thephotoelectric conversion element and the transistor and holds the gatepotential of the transistor; and a second switching element whichcontrols the signal output from the transistor.

In the case where the above configuration of a signal output circuit isemployed, the gate potential of the transistor can be held by turningoff the first switching element; thus, data based on the amount of lightentering the photoelectric conversion element can be held in differentsignal output circuits in different periods. In the n signal outputcircuits, data in different periods (data based on the amount of lightentering the photoelectric conversion element) is held, and then, thesecond switching elements are turned on; thus, signals in differentperiods can be obtained without being affected by the characteristics ofthe photoelectric conversion element.

In other words, according to one embodiment of the present invention, aphotodetector circuit includes a photoelectric conversion element and nsignal output circuits (n is a natural number of 2 or more) connected tothe photoelectric conversion element. The n signal output circuits eachinclude a transistor whose gate potential varies in accordance with theamount of light entering the photoelectric conversion element and whichoutputs a signal corresponding to the gate potential; a first switchingelement which is connected between the photoelectric conversion elementand the transistor and holds the gate potential; and a second switchingelement which controls output of the signal. Gate potentials held in then signal output circuits are based on the amount of light entering thephotoelectric conversion element in different periods. After the gatepotentials are held in the n signal output circuits, signalscorresponding to the gate potentials are output from the n signal outputcircuits.

When the photodetector circuit has the above-described configuration,the photodetector circuit can obtain signals in different periodswithout being affected by characteristics of the photoelectricconversion element.

In the above-described photodetector circuit, by providing a wiringwhich is connected to the second switching elements in the n signaloutput circuits and transmits signals for controlling the operation ofthe second switching elements, the number of wirings necessary forperforming on/off operations of the second switching elements in thesignal output circuits can be reduced. In addition, since signals can beoutput from the n signal output circuits at the same time, the signalscan be obtained in a short period.

Further, in the case where a transistor including an oxide semiconductormaterial in a channel formation region is used as the first switchingelement in the above photodetector circuit, the first switching elementhas extremely low off-state current, and thus can hold the gatepotential of the transistor. Thus, a signal output from the signaloutput circuit is an extremely accurate signal including datacorresponding to the amount of light entering the photoelectricconversion element.

Note that in the case where a transistor including an oxidesemiconductor material in a channel formation region is used as each ofthe second switching element and the transistor in addition to the firstswitching element, elements included in the signal output circuits canbe manufactured in the same steps; thus, time and cost for manufacturingphotodetector circuits can be reduced.

Further, in the case where the above-described photodetector circuit isused for a semiconductor device, with a configuration in which thephotodetector circuits are arranged in a matrix and gate potentials areheld in the n signal output circuits in all the photodetector circuitsarranged in a matrix, and then, n signals corresponding to the gatepotentials are output from the photodetector circuits, signals indifferent periods can be obtained from all the photodetector circuits ina short period.

Specific examples of semiconductor devices include radiation imagingdevices, for example. In the case where the above-describedphotodetector circuit is used for a radiation imaging device, theradiation imaging device includes a radiation source, a scintillatorwhich outputs light by receiving radiation output from the radiationsource, a photodetector mechanism including the photodetector circuitsarranged in a matrix and a photodetector circuit control portion whichcontrols operations of the photodetector circuits, and a detectionsignal comparison portion which compares signals output from thephotodetector circuit control portion. The photodetector mechanism mayhave a structure in which gate potentials are held in the n signaloutput circuits in all the photodetector circuits arranged in a matrix,the n signals corresponding to the gate potentials are output from thephotodetector circuits, and then, the detection signal comparisonportion compares the n signals output from the photodetector circuits.

Examples of semiconductor devices other than radiation imaging devicesare image display devices, for example. In the case where theabove-described photodetector circuit is used for an image displaydevice, the image display device includes a display portion in whichpixels including a display element and a photodetector circuit arearranged in a matrix, a display element control portion which controlsthe operations of the display elements, a photodetector circuit controlportion which controls the operations of the photodetector circuits, andan image signal generation portion which generates image signals byusing signals output from the photodetector circuit control portion.Gate potentials are held in the n signal output circuits in all thephotodetector circuits in the pixels arranged in a matrix and then, then signals corresponding to the gate potentials are output from thephotodetector circuits, and the image signal generation portiongenerates image signals from the n signals output from the photodetectorcircuits.

Further, one embodiment of the present invention is a method foroperating a photodetector circuit which includes a photoelectricconversion element and n signal output circuits (n is a natural numberof 2 or more) connected to the photoelectric conversion element. The nsignal output circuits each include a transistor whose gate potentialvaries in accordance with the amount of light entering the photoelectricconversion element and which outputs a signal corresponding to the gatepotential, a first switching element which is connected between thephotoelectric conversion element and the transistor and which holds thegate potential, and a second switching element which controls the signaloutput from the transistor. The method includes the steps of: holdingpotentials based on the amount of light entering the photoelectricconversion element as gate potentials by turning off the first switchingelements in the n signal output circuits in different periodsindependently of the signal output circuits; and outputting signalscorresponding to the gate potentials by turning on the second switchingelements.

By driving the photodetector circuit by the above-described operationmethod, in the photodetector circuit, signals corresponding to theamount of light entering the photoelectric conversion element indifferent periods can be obtained in a short period without beingaffected by characteristics of the photoelectric conversion element.

Note that in the above-described method for operating the photodetectorcircuit, by performing operations for initializing the gate potentialsin the n signal output circuits at the same time, the gate potentials inthe n signal output circuits can be reset at the same time; thus,signals can be obtained in a short period.

Further, in the above-described method for operating the photodetectorcircuit, by performing the operations of turning on the second switchingelements and the operations of turning off the second switching elementsin the n signal output circuits at the same time, signals can be outputfrom the n signal output circuits at the same time; thus, the signalscan be obtained in a short period.

According to one embodiment of the present invention, a photodetectorcircuit has a configuration in which n output circuits (n is a naturalnumber of 2 or more) are connected to a photoelectric conversionelement, and the output circuit includes a transistor whose outputsignal varies in accordance with the level of a potential generated, afirst switching element which prevents leakage of the potential from theoutput circuit, and a second switching element which controls the outputsignal from the transistor. In the n output circuits, after the signalsare held in different periods (at different timings) in the outputcircuits, the signals are output from the n output circuits.

Thus, a photodetector circuit capable of obtaining signals in differentperiods without being affected by characteristics of a photoelectricconversion element can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A illustrate a configuration of a photodetector circuit and FIG.1B is an operation flow chart of the photodetector circuit;

FIG. 2 is an operation flow chart of a photodetector circuit;

FIG. 3A illustrates a configuration of a photodetector circuit and FIG.3B is an operation flow chart of the photodetector circuit;

FIG. 4 is an operation flow chart of a photodetector circuit;

FIG. 5 illustrates a configuration of a photodetector circuit;

FIG. 6 illustrates a configuration of a photodetector circuit;

FIGS. 7A to 7C each illustrate an operational amplifier circuit;

FIGS. 8A and 8B illustrate a layout of a photodetector circuit;

FIGS. 9A and 9B illustrate a layout of a photodetector circuit;

FIGS. 10A and 10B illustrate a structure of a radiation imaging device;

FIGS. 11A to 11D illustrate operations of a radiation imaging device;

FIG. 12 illustrates a structure of an image display device;

FIG. 13 illustrates a configuration of an image display device;

FIGS. 14A and 14B illustrate operations of an image display device; and

FIG. 15 illustrates a configuration of a photodetector circuit.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described below in detail with reference to theaccompanying drawings. Note that embodiments described below can beimplemented in many different modes, and it is easily understood bythose skilled in the art that modes and details can be variously changedwithout departing from the spirit and scope of the present invention.Therefore, the present invention should not be construed as beinglimited to the following description of the embodiments. In the drawingsfor explaining the embodiments, the same parts or parts having a similarfunction are denoted by the same reference numerals, and description ofsuch parts is not repeated.

Note that in the embodiments described below, “one terminal” of atransistor refers to one of a source electrode and a drain electrode,and “the other terminal” of the transistor refers to the other of thesource electrode and the drain electrode. That is, when one terminal ofthe transistor is the source electrode, the other terminal of thetransistor refers to the drain electrode.

Note that “electrical connection” in this specification corresponds tothe state where current, voltage, or potential can be supplied ortransmitted. Therefore, a state of electrical connection means not onlya state of direct connection but also a state of indirect connectionthrough a circuit element such as a wiring, a resistor, a diode, or atransistor, in which current, voltage, or potential can be supplied ortransmitted.

Unless otherwise specified, in the case of an n-channel transistor, theoff-state current in this specification is a current that flows betweena source electrode and a drain electrode when the potential of a gateelectrode is less than or equal to 0 V with the potential of the sourceelectrode as a reference potential while the potential of the drainelectrode is higher than those of the source electrode and the gateelectrode. Moreover, in the case of a p-channel transistor, theoff-state current in this specification is a current that flows betweena source electrode and a drain electrode when the potential of a gateelectrode is greater than or equal to 0 V with the potential of thesource electrode as a reference potential while the potential of thedrain electrode is lower than those of the source electrode and the gateelectrode.

Embodiment 1

In this embodiment, a configuration and an operation method of aphotodetector circuit are described with reference to FIGS. 1A and 1Band FIG. 2.

<Configuration of Photodetector Circuit>

FIG. 1A shows an example of a circuit diagram illustrating aconfiguration of a photodetector circuit. The photodetector circuitincludes a photoelectric conversion element 100 and two signal outputcircuits (a first signal output circuit 101 and a second signal outputcircuit 102) connected to the photoelectric conversion element 100.

<Photoelectric Conversion Element>

As the photoelectric conversion element 100, a photodiode is illustratedin FIGS. 1A and 1B. The photodiode generates current by irradiation withlight from the outside, and the value of photocurrent varies inaccordance with the intensity of incident light. Note that thephotoelectric conversion element 100 is not limited to a photodiode. Forexample, the photoelectric conversion element 100 may be a variableresistor. The variable resistor can include a pair of electrodes and anamorphous silicon layer having i-type conductivity provided between thepair of electrodes. The i-type amorphous silicon layer can be used in amanner similar to that of a photodiode because the resistance of thei-type amorphous silicon layer varies by irradiation with light.

One of the electrodes of the photoelectric conversion element 100 isconnected to a wiring 111 (also referred to as a wiring PR) and theother of the electrodes of the photoelectric conversion element 100 isconnected to the first signal output circuit 101 and the second signaloutput circuit 102.

Needless to say, one of the electrodes of the photoelectric conversionelement 100 may be connected to the first signal output circuit 101 andthe second signal output circuit 102 and the other of the electrodes ofthe photoelectric conversion element 100 may be connected to the wiring111.

The signal output circuits (the first signal output circuit 101 and thesecond signal output circuit 102) hold potentials including the amountof light entering the photoelectric conversion element 100 as data inthe circuits and output detection signals corresponding to thepotentials to the outside.

In the description of this embodiment, the two signal output circuits(the first signal output circuit 101 and the second signal outputcircuit 102) have the same structure; thus, components included in thesignal output circuits are denoted by the same reference numerals. Forexample, both a transistor in the first signal output circuit 101 and atransistor in the second signal output circuit 102 are referred to as a“transistor 120”.

<Detection Circuit>

The first signal output circuit 101 includes the following: a transistor120 whose gate potential varies in accordance with the amount of lightentering the photoelectric conversion element 100 and which outputs asignal corresponding to the gate potential; a first switching element121 which is connected between the photoelectric conversion element 100and the transistor 120, controls the connection state therebetween, andholds a potential applied to a gate of the transistor 120; and a secondswitching element 122 which controls the signal output from thetransistor 120.

The gate of the transistor 120 in the first signal output circuit 101 isconnected to a wiring 112 (also referred to as a wiring FD1), one of asource and a drain of the transistor 120 in the first signal outputcircuit 101 is connected to a wiring 113 (also referred to as a wiringVR), and the other of the source and the drain of the transistor 120 inthe first signal output circuit 101 is connected to one of electrodes ofthe second switching element 122.

Since the first switching element 121 in the first signal output circuit101 holds the potential applied to the gate of the transistor 120, it ispreferable that the first switching element 121 have extremely lowleakage current in an off state.

As an example of a switching element which has low leakage current in anoff state, a transistor which includes an oxide semiconductor materialin a channel formation region can be given.

The above oxide semiconductor material preferably contains at leastindium (In) or zinc (Zn). In particular, In and Zn are preferablycontained. The oxide semiconductor material preferably contains, inaddition to In and Zn, gallium (Ga) serving as a stabilizer that reducesvariations in electrical characteristics among transistors including theoxide semiconductor material. Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium(Lu) may be contained.

The bandgap of a film using an oxide semiconductor material is greaterthan or equal to 3.0 eV (electron volts), which is much wider than thebandgap of silicon (1.1 eV).

The off-resistance of a transistor (resistance between a source and adrain of the transistor in an off state) is inversely proportional tothe concentration of carriers thermally excited in a channel formationregion. Since the bandgap of silicon is 1.1 eV even in a state wherethere is no carrier caused by a donor or an acceptor (i.e., even in thecase of an intrinsic semiconductor), the concentration of thermallyexcited carriers at room temperature (300 K) is approximately 1×10⁻⁷cm⁻³.

The bandgap of a film using an oxide semiconductor material is generallyas wide as 3.0 eV or more as described above, and the concentration ofthermally excited carriers in a film with a bandgap of 3.2 eV, forexample, is approximately 1×10⁻⁷ cm⁻³. When the electron mobility is thesame, the resistivity is inversely proportional to the carrierconcentration, and thus the resistivity of the semiconductor with abandgap of 3.2 eV is 18 orders of magnitude higher than that of silicon.

Since a transistor that uses such a wide bandgap oxide semiconductormaterial in a channel formation region can achieve extremely lowoff-state current.

Further, the transistor is used as the first switching element 121, andafter the gate potential of the transistor 120 varies in accordance withthe amount of light entering the photoelectric conversion element 100,the first switching element 121 is turned off, whereby the gatepotential of the transistor 120 can be held in the wiring 112 for a longtime.

Although a transistor including an oxide semiconductor material in achannel formation region is described above as an example of the firstswitching element 121, another switching element having low off-statecurrent may be used. For example, a transistor using a magnetoresistanceeffect (also referred to as a spin transistor or the like), a transistorusing a ferroelectric material for a gate insulating film (also referredto as a ferroelectric transistor or the like), or the like can be used.

A signal corresponding to the gate potential of the transistor 120 isoutput from the drain (or source) of the transistor 120. Accordingly,the signal can be regarded as a “signal including the amount of lightentering the photoelectric conversion element 100 as data”.

One of a source and a drain of the transistor including an oxidesemiconductor material in a channel formation region, which is used asthe first switching element 121, is connected to the other of theelectrodes of the photoelectric conversion element 100, the other of thesource and the drain of the transistor including an oxide semiconductormaterial in a channel formation region is connected to the gate of thetransistor 120, and a gate of the transistor including an oxidesemiconductor material in a channel formation region is connected to awiring 114 (also referred to as a wiring TX1).

Although in this embodiment and the like, a transistor including anoxide semiconductor material in a channel formation region is used asthe first switching element 121, the first switching element 121 is notlimited to a transistor as long as it is an element capable of switchingon and off of a connection state (conduction state), and a variety ofknown techniques can be used.

One of a source and a drain of the second switching element 122 in thefirst signal output circuit 101 is connected to the other of the sourceand the drain of the transistor 120, the other of the source and thedrain of the second switching element 122 in the first signal outputcircuit 101 is connected to a wiring 115 (also referred to as a wiringOUT), and a gate of the second switching element 122 in the first signaloutput circuit 101 is connected to a wiring 116 (also referred to as awiring SE1).

In the case where a transistor is used as the second switching element122 as illustrated in FIG. 1A, by setting Vgs (a voltage differencebetween a gate and a source when the source is used as a reference) ofthe transistor to a voltage sufficiently higher than the thresholdvoltage, a signal output from the transistor 120 is output to the wiring115 (OUT).

Note that an integrator circuit may be connected to the wiring 115(OUT). Connecting the integrator circuit to the wiring 115 (OUT)increases S/N, enabling detection of weaker light. A specificconfiguration example of the integrator circuit will be described inEmbodiment 2.

The second signal output circuit 102 includes the following: thetransistor 120 whose gate potential varies in accordance with the amountof light entering the photoelectric conversion element 100 and whichoutputs a signal corresponding to the gate potential; the firstswitching element 121 which is connected between the photoelectricconversion element 100 and the transistor 120, controls the connectionstate therebetween, and holds a potential applied to a gate of thetransistor 120; and the second switching element 122 which controls thesignal output from the transistor 120.

The gate of the transistor 120 in the second signal output circuit 102is connected to a wiring 132 (also referred to as a wiring FD2), one ofa source and a drain of the transistor 120 in the second signal outputcircuit 102 is connected to the wiring 113 (also referred to as thewiring VR), and the other of the source and the drain of the transistor120 in the second signal output circuit 102 is connected to one ofelectrodes of the second switching element 122.

The wiring connected to the one of the source and the drain of thetransistor 120 in the second signal output circuit 102 is the same asthe wiring 113 in the first signal output circuit 101.

Since the first switching element 121 in the second signal outputcircuit 102 holds the potential applied to the gate of the transistor120, the first switching element 121 preferably has extremely lowoff-state current, and for example, a transistor including an oxidesemiconductor material in a channel formation region can be used. Forthe description of the transistor including an oxide semiconductormaterial in a channel formation region, the above “description of thefirst signal output circuit 101”can be referred to.

Since the transistor including an oxide semiconductor material in achannel formation region has extremely low off-state current, thetransistor is used as the first switching element 121, and after thegate potential of the transistor 120 varies in accordance with theamount of light entering the photoelectric conversion element 100, thefirst switching element 121 is turned off, whereby the gate potential ofthe transistor 120 can be held in the wiring 132 for a long time.

Further, a signal (hereinafter, the signal output from the second signaloutput circuit 102 is also referred to as a second signal) correspondingto the gate potential of the transistor 120 is output from the drain (orsource) of the transistor 120.

One of a source and a drain of the transistor including an oxidesemiconductor material in a channel formation region, which is used asthe first switching element 121, is connected to the other of theelectrodes of the photoelectric conversion element 100, the other of thesource and the drain of the transistor including an oxide semiconductormaterial in a channel formation region is connected to the gate of thetransistor 120, and a gate of the transistor including an oxidesemiconductor material in a channel formation region is connected to awiring 134 (also referred to as a wiring TX2).

Although in this embodiment and the like, a transistor including anoxide semiconductor material in a channel formation region is used asthe first switching element 121, the first switching element 121 is notlimited to a transistor as long as it is an element capable of switchingon and off of a connection state (conduction state).

One of a source and a drain of the second switching element 122 in thesecond signal output circuit 102 is connected to the other of the sourceand the drain of the transistor 120, the other of the source and thedrain of the second switching element 122 in the second signal outputcircuit 102 is connected to the wiring 115 (also referred to as thewiring OUT), and the gate of the second switching element 122 in thesecond signal output circuit 102 is connected to a wiring 136 (alsoreferred to as a wiring SE2).

The wiring connected to the other of the source and the drain of thesecond switching element 122 in the second signal output circuit 102 isthe same as the wiring 115 in the first signal output circuit 101.

When the photodetector circuit has the above-described structure, byturning on the first switching elements 121 in the signal outputcircuits at different timings, the amount of light entering thephotoelectric conversion element 100 at different timings can bedetected. By turning off the first switching elements 121, the data canbe held as gate potentials; thus, for example, even when light enteringthe photoelectric conversion element 100 in a first period is lightgenerating a composite signal, a potential including the light as datais held in the first signal output circuit 101, light generating a noisesignal is detected in a second period, and a potential including thelight as data is held in the second signal output circuit 102, whereby asignal necessary for generation of an essential signal can be obtainedfrom the photodetector circuit.

In this embodiment, it is preferable that the transistor 120 have highmobility because the transistor 120 amplifies an electrical signalgenerated by the photoelectric conversion element 100.

As an example of the transistor 120 having high mobility, a thin filmtransistor including amorphous silicon, microcrystalline silicon,polycrystalline silicon, single crystal silicon, or the like in achannel formation region can be given.

Further, the transistor 120 needs low off-state current characteristicsin order to prevent output of an unnecessary potential to the wiring 113(VR). For these reasons, it is also effective to use a transistor usingan oxide semiconductor material, which achieves both high mobility andlow off-state current in a channel formation region, as the transistor120.

In this embodiment, the second switching element 122 preferably has highmobility because of controlling output of a signal from the signaloutput circuit.

As an example of the second switching element 122 having high mobility,a thin film transistor including amorphous silicon, microcrystallinesilicon, polycrystalline silicon, single crystal silicon, or the like ina channel formation region can be given.

Further, the second switching element 122 needs low off-state currentcharacteristics in order to prevent output of an unnecessary potentialto the wiring 115 (OUT). For these reasons, it is also effective to usea transistor using an oxide semiconductor material, which achieves bothhigh mobility and low off-state current in a channel formation region,as the second switching element 122.

The use of transistors including an oxide semiconductor material in achannel formation region as all the components (the transistor 120, thefirst switching element 121, and the second switching element 122) ineach of the signal output circuits can simplify the manufacturingprocess of the signal output circuits.

When a semiconductor material capable of providing higher mobility thanan oxide semiconductor material, such as polycrystalline or singlecrystal silicon, is used for the channel formation regions of thetransistor 120 and the second switching element 122, data can be readfrom the signal output circuit at high speed.

Connecting a capacitor to the wiring 115 (OUT) is effective instabilizing the potential of the wiring 115 (OUT).

In FIG. 1A, the transistor 120 and the second switching element 122 areconnected in series in this order between the wiring 113 (VR) and thewiring 115 (OUT); alternatively, the transistor 120 and the secondswitching element 122 may be connected in reverse.

In FIG. 1A, the transistor 120 has a gate only on one side of asemiconductor layer; however, the transistor 120 may have a pair ofgates placed so that the semiconductor layer is sandwiched therebetween.When the transistor 120 has a pair of gates placed so that thesemiconductor layer is sandwiched therebetween, one of the gates canfunction as a front gate to which the potential of the wiring 112 (orthe wiring 132) is applied, and the other gate can function as abackgate that controls the threshold voltage or the like of thetransistor 120. In this case, the potential applied to the other gatepreferably ranges from −20 V to +2 V with reference to the sourcepotential. If a change in the threshold voltage of the transistor 120does not adversely affect the operation of the signal output circuitwhen the potential applied to the other gate varies in the above range,the other gate may be electrically isolated (floating).

The above is the description of the configuration of the circuits in thephotodetector circuit. A layout example of the configuration of thecircuits illustrated in FIG. 1A, which is described in this embodiment,will be described in Embodiment 4.

Although the photodetector circuit described in this embodiment includesone photoelectric conversion element and two signal output circuitsconnected to the photoelectric conversion element, it may include nsignal output circuits (n is a natural number of 2 or more). Forexample, as illustrated in FIG. 15, a structure in which onephotoelectric conversion element and four signal output circuits (thefirst signal output circuit 101, the second signal output circuit 102, athird signal output circuit 103, and a fourth signal output circuit 104)are provided may be used. Since one photoelectric conversion element isshared by four signal output circuits, sharing wirings and a large-areaphotoelectric conversion element can be achieved. Alternatively, in thecase where the area of the photoelectric conversion element does notneed to be increased, the area of the photodetector circuit can bereduced.

Further, the configuration of the photodetector circuit described inthis embodiment may be a configuration in which a transistor 501 isadded to each of the first signal output circuit 101 and the secondsignal output circuit 102 as illustrated in FIG. 5. A gate of thetransistor is electrically connected to the wiring 111 (PR), one of asource and a drain of the transistor is electrically connected to thewiring 112 (FD1) (or the wiring 132 (FD2)), and the other of the sourceand the drain of the transistor is electrically connected to a wiring502 a (or a wiring 502 b). The one of the electrodes of thephotoelectric conversion element 100 is electrically connected to awiring 503. Here, the wiring 503 is a signal line (low potential line)for applying a reverse bias to the photoelectric conversion element 100.Further, the wiring 502 a and the wiring 502 b are signal lines (highpotential lines) for resetting the wiring 112 (FD1) (or the wiring 132(FD2)) to a high potential.

The transistor 501 functions as a reset transistor for resetting thewiring 112 (FD1) (or the wiring 132 (FD2)). Accordingly, unlike in thedetection circuit in FIG. 1A, the reset operation using thephotoelectric conversion element 100 is not performed, and a reversebias is always applied to the photoelectric conversion element 100. Thewiring 112 (FD1) and the wiring 132 (FD2) can be reset by setting thepotential of the wiring 111 (PR) high.

The transistor 501 can be formed using a silicon semiconductor such asamorphous silicon, microcrystalline silicon, polycrystalline silicon, orsingle crystal silicon; however, when leakage current is large, thecharge accumulation portion cannot hold charge long enough. For thisreason, like the transistor 120, it is preferable to use a transistorincluding a semiconductor layer (at least a channel formation region)formed using an oxide semiconductor material, which achieves extremelylow off-state current.

<Operation Flow Chart of Photodetector Circuit>

Next, an operation flow chart of the photodetector circuit illustratedin FIG. 1A will be described with reference to FIG. 1B.

In FIGS. 1B, 114S, 112S, and 116S correspond to potentials of the wiring114 (TX1), the wiring 112 (FD1), and the wiring 116 (SE1) in the firstsignal output circuit 101, and 134S, 132S, and 136S correspond topotentials of the wiring 134 (TX2), the wiring 132 (FD2), and the wiring136 (SE2) in the second signal output circuit 102. Further, 11S and 115Scorrespond to potentials of the wiring 111 (PR) and the wiring 115 (OUT)which are used in common in the first signal output circuit 101 and thesecond signal output circuit 102. Note that the potential of the wiring113 (VR) is fixed at a low level.

First, at a time T1, the potential (the signal 11S) of the wiring 111 isset high and the potential (the signal 114S) of the wiring 114 (TX1) inthe first signal output circuit 101 is set high (i.e., a reset operationstarts).

Thus, a forward bias is applied to the photoelectric conversion element100 and the potential (the signal 112S) of the wiring 112 (FD1) in thefirst signal output circuit 101 becomes high. Note that the potential(the signal 115S) of the wiring 115 (OUT) is precharged to high.

Next, at a time T2, the potential (the signal 11S) of the wiring 111(PR) is set low and the potential (the signal 114S) of the wiring 114(TX1) in the first signal output circuit 101 is set high (i.e., thereset operation finishes and a potential generation operation starts).

Thus, reverse current flows through the photoelectric conversion element100 in accordance with the amount of light entering the photoelectricconversion element 100, and the potential (the signal 112S) of thewiring 112 (FD1) in the first signal output circuit 101 starts to belowered.

Since the amount of reverse current increases when the photoelectricconversion element 100 is irradiated with light, the speed of decreasein the potential (the signal 112S) of the wiring 112 (FD) in the firstsignal output circuit 101 changes in accordance with the amount ofincident light. In other words, the channel resistance between thesource and the drain of the transistor 120 in the first signal outputcircuit 101 changes in accordance with the amount of light entering thephotoelectric conversion element 100.

Then, at a time T3, the potential (the signal 114S) of the wiring 114(TX1) in the first signal output circuit 101 is set low (i.e., thepotential generation operation finishes).

Since the first switching element 121 in this embodiment and the like isa transistor which includes an oxide semiconductor material in a channelformation region as described above and thus has extremely low off-statecurrent, the potential applied to the gate of the transistor 120 in thefirst signal output circuit 101 can be held in the wiring 112 (FD1)until an output operation is performed later.

Note that when the potential (the signal 114S) of the wiring 114 (TX1)in the first signal output circuit 101 is set low, the potential of thewiring 112 (FD) sometimes changes because of parasitic capacitancebetween the wiring 114 (TX1) and the wiring 112 (FD1) in the firstsignal output circuit 101. A large amount of potential change makes itimpossible to obtain an accurate amount of charge generated by thephotoelectric conversion element 100 during the potential generationoperation.

Examples of effective measures to reduce the amount of potential changeinclude reducing the capacitance between the gate and the source (orbetween the gate and the drain) of the transistor used as the firstswitching element 121, increasing the gate capacitance of the transistor120, and providing a storage capacitor to connect the wiring 112 (FD1)in the first signal output circuit 101. Note that in FIG. 1B, thepotential change can be ignored by the adoption of these measures.

Next, also in the second signal output circuit 102, in order to hold apotential including the amount of light entering the photoelectricconversion element 100 as data in the second signal output circuit 102,a “reset operation” and a “potential generation operation” are performedin a manner similar to those of the above operations in the first signaloutput circuit 101. Thus, the potential including the amount of lightentering the photoelectric conversion element 100 as data can be held inthe wiring 132 until an output operation is performed later (theoperations from the time T4 to the time T6 correspond to the resetoperation and the potential generation operation).

Then, at a time 17, when the potential (the signal 116S) of the wiring116 (SE1) in the first signal output circuit 101 is set high (i.e., theoutput operation starts), current corresponding to the gate potential ofthe transistor 120 flows between the source and the drain of the secondswitching element 122, so that the potential (the signal 115S) of thewiring 115 (OUT) decreases. Note that precharge of the wiring 115 (OUT)is terminated before the time T7.

Here, the speed of decrease in the potential (the signal 115S) of thewiring 115 (OUT) depends on the channel resistance between the sourceand the drain of the transistor 120 in the first signal output circuit101. That is, the speed of decrease in the potential (the signal 115S)of the wiring 115 (OUT) changes in accordance with the amount of lightentering the photoelectric conversion element 100 during the potentialgeneration operation in the first signal output circuit 101.

Then, at a time T8, when the potential (the signal 116S) of the wiring116 (SE1) in the first signal output circuit 101 is set low (i.e., theoutput operation finishes), current flowing between the source and thedrain of the second switching element 122 is stopped and the potential(the signal 115S) of the wiring 115 (OUT) becomes a fixed value.

Here, the fixed value varies in accordance with the amount of lightentering the photoelectric conversion element 100 during the potentialgeneration operation in the first signal output circuit 101. Thus, byobtaining the potential (the signal 115S) of the wiring 115 (OUT), theamount of light entering the photoelectric conversion element 100 duringthe potential generation operation in the first signal output circuit101 can be found out. That is, a signal output from the first signaloutput circuit 101 after the output operation is a detection signal inthe first signal output circuit 101.

More specifically, in the case where the amount of light entering thephotoelectric conversion element 100 is large, in the first signaloutput circuit 101, the potential (the signal 112S) of the wiring 112(FD1) becomes lower and the gate potential of the transistor 120 becomeslower; thus, the speed of decrease in the potential (the signal 115S) ofthe wiring 115 (OUT) becomes lower. As a result, the potential of thewiring 115 (OUT) becomes higher.

Alternatively, in the case where the amount of light entering thephotoelectric conversion element 100 is small, in the first signaloutput circuit 101, the potential (the signal 112S) of the wiring 112(FD1) becomes higher and the gate potential of the transistor 120becomes higher, thus, the speed of decrease in the potential (the signal1155S) of the wiring 115 (OUT) becomes higher. As a result, thepotential of the wiring 115 (OUT) becomes lower.

Next, the wiring 115 (OUT) is precharged.

Also in the second signal output circuit 102, an “output operation” isperformed in a manner similar to that of the above operation in thefirst signal output circuit 101. Thus, a detection signal in the secondsignal output circuit 102 is obtained (the operations between the timeT9 and the time T10 correspond to the output operation).

As described above, potentials (data) based on the amount of lightentering the photoelectric conversion element 100 in different periods(a potential generation operation period in the first signal outputcircuit 101 and a potential generation operation period in the secondsignal output circuit 102) can be held in the signal output circuits byusing the transistor 120 and the first switching element 121. Further,after the potentials are held in all the signal output circuits, adetection signal is obtained from each of the signal output circuits byusing the second switching element in the signal output circuit; thus,detection signals in different periods can be obtained without beingaffected by characteristics of the photoelectric conversion element.

The above is the description of the operation flow chart of thephotodetector circuit of this embodiment.

<Different Operation Flow Chart of Photodetector Circuit>

Note that the operation flow chart of the photodetector circuitdescribed in FIG. 1A may be an operation flow chart different from theabove operation flow chart described using FIG. 1B. The operation flowchart different from the above-described operation flow chart isdescribed below using FIG. 2.

First, at a time T1, the potential (the signal 111S) of the wiring 111is set high and the potential (the signal 114S) of the wiring 114 (TX1)in the first signal output circuit 101 and the potential (the signal134S) of the wiring 134 (TX2) in the second signal output circuit 102are set high (i.e., a reset operation starts).

In the operation flow chart described in FIG. 1B, reset operations areperformed in the first signal output circuit 101 and the second signaloutput circuit 102 in separate steps. By performing reset operations inthe first signal output circuit 101 and the second signal output circuit102 at the same time as illustrated in FIG. 2, a period from the resetoperation start to the output operation end (period from the time T1 tothe time T10) can be shortened, so that detection signals in differentperiods can be obtained in a short period.

Note that since the following operation flow chart is the same as thatin the above operation flow chart described using FIG. 1B except for theoperation flow chart between the time T4 and the time T5, the operationflow chart described using FIG. 1B can be referred to for the followingoperation flow chart.

The above is the description of the different operation flow chart ofthe photodetector circuit.

In the case of using the above-described operation, it is preferablethat capacitance in the wiring 112 (FD1) and the wiring 132 (FD2) belarger than wiring capacitances between the photoelectric conversionelement 100 and the first switching element 121 in the first signaloutput circuit 101 and between the photoelectric conversion element 100and the first switching element 121 in the second signal output circuit102.

Embodiment 2

In this embodiment, a photodetector circuit whose structure andoperation method are different from those in Embodiment 1 will bedescribed with reference to FIGS. 3A and 3B and FIG. 4.

<Configuration of Photodetector Circuit>

FIG. 3A shows an example of a circuit diagram illustrating aconfiguration of a photodetector circuit. The photodetector circuitincludes, as in Embodiment 1, the photoelectric conversion element 100and two signal output circuits (a first signal output circuit 301 and asecond signal output circuit 302) connected to the photoelectricconversion element 100.

<Photoelectric Conversion Element>

Although a photodiode is used as the photoelectric conversion element100 as in Embodiment 1, the photoelectric conversion element 100 is notlimited to a photodiode.

One of the electrodes of the photoelectric conversion element 100 isconnected to the wiring 111 (PR) and the other of the electrodes of thephotoelectric conversion element 100 is connected to the first signaloutput circuit 301 and the second signal output circuit 302.

The signal output circuits (the first signal output circuit 301 and thesecond signal output circuit 302) hold potentials including the amountof light entering the photoelectric conversion element 100 as data inthe circuits and output detection signals corresponding to thepotentials (data) to the outside.

<Detection Circuit>

Although this embodiment is similar to Embodiment 1 in that the firstsignal output circuit 301 and the second signal output circuit 302illustrated in FIG. 3A each include the transistor 120, the firstswitching element 121, and the second switching element 122 ascomponents, different points are as follows: a wiring which controls theoperation state of the second switching element 122 is shared by thefirst signal output circuit 301 and the second signal output circuit302; and different wirings are used in the first signal output circuit301 and the second signal output circuit 302 to output a detectionsignal.

Specifically, in the photodetector circuit illustrated in FIG. 1A, thesecond switching element 122 in the first signal output circuit 101 isconnected to the wiring 116 (SE1), and the second switching element 122in the second signal output circuit 102 is connected to the wiring 136(SE2).

In contrast, in the photodetector circuit illustrated in FIG. 3A, thesecond switching element 122 in the first signal output circuit 301 andthe second switching element 122 in the second signal output circuit 302are both connected to a wiring 316 (SE).

Further, in the photodetector circuit illustrated in FIG. 1A, the secondswitching element 122 in the first signal output circuit 101 and thesecond switching element 122 in the second signal output circuit 102 areboth connected to the wiring 115 (OUT).

In contrast, in the photodetector circuit illustrated in FIG. 3A, thesecond switching element 122 in the first signal output circuit 301 isconnected to a wiring 315 (OUT1) and the second switching element 122 inthe second signal output circuit 302 is connected to a wiring 335(OUT2).

When the photodetector circuit has the above-described structure,detection signals can be output from the first signal output circuit 301and the second signal output circuit 302 at the same time; thus,detection signals can be obtained in a short period.

Note that the configuration of the photodetector circuit described inthis embodiment may be a configuration in which a transistor 601 isadded to each of the first signal output circuit 301 and the secondsignal output circuit 302 as illustrated in FIG. 6. A gate of thetransistor is electrically connected to the wiring 111 (PR), one of asource and a drain of the transistor is electrically connected to thewiring 112 (FD1) (or the wiring 132 (FD2)), the other of the source andthe drain of the transistor is electrically connected to a wiring 602 a(or a wiring 602 b), and the one of the electrodes of the photoelectricconversion element 100 is electrically connected to a wiring 603. Here,the wiring 603 is a signal line (low potential line) for always applyinga reverse bias to the photoelectric conversion element 100. Further, thewiring 602 a and the wiring 602 b are signal lines (high potentiallines) for resetting the wiring 112 (FD1) (or the wiring 132 (FD2)) to ahigh potential.

The transistor 601 functions as a reset transistor for resetting thewiring 112 (FD1) (or the wiring 132 (FD2)). Accordingly, unlike in thedetection circuit in FIG. 3A, the reset operation using thephotoelectric conversion element 100 is not performed, and a reversebias is always applied to the photoelectric conversion element 100. Thewiring 112 (FD1) and the wiring 132 (FD2) can be reset by setting thepotential of the wiring 111 (PR) high.

The transistor 601 can be formed using a silicon semiconductor such asamorphous silicon, microcrystalline silicon, polycrystalline silicon, orsingle crystal silicon; however, when leakage current is large, thecharge accumulation portion cannot hold charge long enough. For thisreason, like the transistor 120, it is preferable to use a transistorincluding a semiconductor layer (at least a channel formation region)formed using an oxide semiconductor material, which achieves extremelylow off-state current.

<Operation Flow Chart of Photodetector Circuit>

Next, an operation flow chart of the photodetector circuit illustratedin FIG. 3A will be described with reference to FIG. 3B.

First, as in the operation flow chart of the photodetector circuitdescribed in Embodiment 1, reset operations and potential generationoperations are performed in the first signal output circuit 301 and thesecond signal output circuit 302 from the time T1 to the time T6.

Next, at the time T7, output operations are performed in the firstsignal output circuit 301 and the second signal output circuit 302.Although the output operations are sequentially performed in the firstsignal output circuit 101 and the second signal output circuit 102 inEmbodiment 1, in the operation flow chart of the photodetector circuitin this embodiment, the output operations are performed in the firstsignal output circuit 301 and the second signal output circuit 302 at atime (the potential (the signal 316S) of the wiring 316 (SE) is sethigh) as illustrated in FIG. 3B.

Thus, current corresponding to the gate potential of the transistor 120flows between the source and the drain of the second switching element122 in each of the first signal output circuit 301 and the second signaloutput circuit 302, whereby the potential (the signal 315S) of thewiring 315 (OUT1) and the potential (the signal 335S) of the wiring 335(OUT2) are decreased.

Then, at the time T8, when the potential (the signal 316S) of the wiring316 (SE) in the first signal output circuit 301 is set low (i.e., theoutput operation finishes), current flowing between the source and thedrain of the second switching element 122 in each of the first signaloutput circuit 301 and the second signal output circuit 302 is stopped,so that the potential (the signal 315S) of the wiring 315 (OUT1) whichserves as a transmission path of a detection signal output from thefirst signal output circuit 301 and the potential (the signal 335S) ofthe wiring 335 (OUT2) which serves as a transmission path of a detectionsignal output from the second signal output circuit each have a fixedvalue.

As illustrated in FIG. 3A, the wiring (wiring 316 (SE)) which controlsthe operation state of the second switching element 122 is shared by thefirst signal output circuit 301 and the second signal output circuit 302and different wirings (the wiring 315 (OUT1) and the wiring 335 (OUT2))are used in the first signal output circuit 301 and the second signaloutput circuit 302 as wirings for outputting detection signals, so thatoutput of a detection signal from the first signal output circuit 301and output of a detection signal from the second signal output circuit302 can be performed at a time; thus, detection signals in differentperiods can be obtained in a short period.

The above is the description of the operation flow chart of thephotodetector circuit in this embodiment.

<Different Operation Flow Chart of Photodetector Circuit>

Note that the operation flow chart of the photodetector circuitdescribed in FIG. 3A may be an operation flow chart different from theabove operation flow chart described using FIG. 3B. The operation flowchart different from the above-described operation flow chart isdescribed below using FIG. 4.

First, at a time T1, the potential (the signal 111S) of the wiring 111is set high and the potential (the signal 114S) of the wiring 114 (TX1)in the first signal output circuit 301 and the potential (the signal134S) of the wiring 134 (TX2) in the second signal output circuit 302are set high (i.e., a reset operation starts).

In the operation flow chart described in FIG. 3B, reset operations areperformed in the first signal output circuit 301 and the second signaloutput circuit 302 in separate steps. By performing reset operations inthe first signal output circuit 301 and the second signal output circuit302 at the same time as illustrated in FIG. 4, a period from the resetoperation start to the output operation end (period from the time T1 tothe time T8) can be shortened, so that detection signals in differentperiods can be obtained in a short period.

Note that since the following operation flow chart is the same as thatin the above operation flow chart described using FIG. 3B except for theoperation flow chart between the time T4 and the time T5, the operationflow chart described using FIG. 3B can be referred to for the followingoperation flow chart.

The above is the description of the different operation flow chart ofthe photodetector circuit.

Embodiment 3

In this embodiment, examples of the configuration of an integratorcircuit used to be connected to the wiring 115 (OUT) in Embodiment 1,and the wiring 315 (OUT1) and the wiring 335 (OUT2) in Embodiment 2.

FIG. 7A illustrates an integrator circuit including an operationalamplifier circuit (also referred to as an op-amp). An inverting inputterminal of the operational amplifier circuit is connected to the wiring115 (OUT), the wiring 315 (OUT1), and the wiring 335 (OUT2) through aresistor R. A non-inverting input terminal of the operational amplifiercircuit is grounded. An output terminal of the operational amplifiercircuit is connected to the inverting input terminal of the operationalamplifier circuit through a capacitor C.

Here, the operational amplifier circuit is assumed to be an idealoperational amplifier circuit. In other words, it is assumed that inputimpedance is infinite (the input terminals draw no current). Since thepotential of the non-inverting input terminal and the potential of theinverting input terminal are equal in a steady state, the potential ofthe inverting input terminal can be considered as a ground potential.

Relations (1), (2), and (3) are satisfied, where Vi is the potential ofeach of the wiring 115 (OUT), the wiring 315 (OUT1), and the wiring 335(OUT2), Vo is the potential of the output terminal of the operationalamplifier circuit, i1 is a current flowing through the resistor R, andi2 is a current flowing through the capacitor C.

Vi=i1·R  (1)

i2=C·dVo/dt  (2)

i1+i2=0  (3)

Here, when charge in the capacitor C is discharged at the time t=0, thepotential Vo of the output terminal of the operational amplifier circuitat the time t=t is expressed by (4).

Vo=−(1/CR)∫Vidt  (4)

In other words, with a longer time t (integral time), the potential (Vi)to be read can be raised and output as the detection signal Vo.Moreover, lengthening of the time t corresponds to averaging of thermalnoise or the like and can increase S/N of the detection signal Vo.

In a real operational amplifier circuit, a bias current flows even whena signal is not input to the input terminals, so that an output voltageis generated at the output terminal and charge is accumulated in thecapacitor C. It is therefore effective to connect a resistor in parallelwith the capacitor C so that the capacitor C can be discharged.

FIG. 7B illustrates an integrator circuit including an operationalamplifier circuit having a structure different from that in FIG. 7A. Aninverting input terminal of the operational amplifier circuit isconnected to the wiring 115 (OUT), the wiring 315 (OUT1), and the wiring335 (OUT2), through a resistor R and a capacitor C1. A non-invertinginput terminal of the operational amplifier circuit is grounded. Anoutput terminal of the operational amplifier circuit is connected to theinverting input terminal of the operational amplifier circuit through acapacitor C2.

Here, the operational amplifier circuit is assumed to be an idealoperational amplifier circuit. In other words, it is assumed that inputimpedance is infinite (the input terminals draw no current). Since thepotential of the non-inverting input terminal and the potential of theinverting input terminal are equal in a steady state, the potential ofthe inverting input terminal can be considered as a ground potential.

Relations (5), (6), and (7) are satisfied, where Vi is the potential ofeach of the wiring 115 (OUT), the wiring 315 (OUT1), and the wiring 335(OUT2), Vo is the potential of the output terminal of the operationalamplifier circuit, i1 is a current flowing through the resistor R andthe capacitor C1, and i2 is a current flowing through the capacitor C2.

Vi=(1/C1)∫i1dt+i1·R  (5)

i2=C2·dVo/dt  (6)

i1+i2=0  (7)

Here, assuming that charge in the capacitor C2 is discharged at the timet=0, the potential Vo of the output terminal of the operationalamplifier circuit at the time t=t is expressed by (9) when (8) is met,which corresponds to a high-frequency component, and (11) when (10) ismet, which corresponds to a low-frequency component.

Vo<<dVo/dt  (8)

Vo=−(1/C2R)∫Vidt  (9)

Vo>>dVo/dt  (10)

Vo=−C1/C2·Vi  (11)

In other words, by appropriately setting the capacitance ratio of thecapacitor C1 to the capacitor C2, the potential (Vi) to be read can beraised and output as the detection signal Vo. Further, a high-frequencynoise component of the input signal can be averaged by time integration,and S/N of the detection signal Vo can be increased.

In a real operational amplifier circuit, a bias current flows even whena signal is not input to the input terminals, so that an output voltageis generated at the output terminal and charge is accumulated in thecapacitor C2. It is thus effective to connect a resistor in parallelwith the capacitor C2 so that the capacitor C2 can be discharged.

FIG. 7C illustrates an integrator circuit including an operationalamplifier circuit having a structure different from those in FIGS. 7Aand 7B. A non-inverting input terminal of the operational amplifiercircuit is connected to the wiring 115 (OUT), the wiring 315 (OUT1), andthe wiring 335 (OUT2) through a resistor R and is grounded through acapacitor C. An output terminal of the operational amplifier circuit isconnected to an inverting input terminal of the operational amplifiercircuit. The resistor R and the capacitor C constitute a CR integratorcircuit. The operational amplifier circuit is a unity gain buffer.

A relation (12) holds, where Vi is the potential of each of the wiring115 (OUT), the wiring 315 (OUT1), and the wiring 335 (OUT2) and Vo isthe potential of the output terminal of the operational amplifiercircuit. Although Vo is saturated at the value of Vi, a noise componentincluded in the input signal Vi can be averaged by the CR integratorcircuit, and as a result, S/N of the detection signal Vo can beincreased.

Vo=(1/CR)∫Vidt  (12)

The above are the examples of the configuration of the integratorcircuit used to be connected to each of the wiring 115 (OUT), the wiring315 (OUT1), and the wiring 335 (OUT2). Connecting the above-describedintegrator circuit to the wiring 115 (OUT), the wiring 315 (OUT1), andthe wiring 335 (OUT2) increases S/N of the detection signal and enablesweaker light to be detected; thus, a more accurate image signal can begenerated in the semiconductor device.

Embodiment 4

In this embodiment, an example of the layout of the photodetectorcircuit in FIG. 1A and FIG. 3A described in Embodiment 1 will bedescribed with reference to FIGS. 8A and 8B and FIGS. 9A and 9B.

<Example of Layout of Photodetector Circuit in FIG. 1A>

FIG. 8A is a top view of the photodetector circuit illustrated in FIG.1A, and FIG. 8B is a cross-sectional view along the dashed-dotted lineA1-A2 in FIG. 8A.

The photodetector circuit includes, over a substrate 860 on which aninsulating film 861 is formed, a conductive film 811 serving as thewiring 111 (PR), a conductive film 812 serving as the wiring 112 (FD1)in the first signal output circuit 101, a conductive film 832 serving asthe wiring 132 (FD2) in the second signal output circuit 102, aconductive film 813 serving as the wiring 113 (VR), a conductive film814 serving as the wiring 114 (TX1) in the first signal output circuit101, a conductive film 834 serving as the wiring 134 (TX2) in the secondsignal output circuit 102, a conductive film 815 serving as the wiring115 (OUT), a conductive film 816 serving as the wiring 116 (SE1) in thefirst signal output circuit 101, and a conductive film 836 serving asthe wiring 136 (SE2) in the second signal output circuit 102.

The photoelectric conversion element 100 includes a p-type semiconductorfilm 801, an i-type semiconductor film 802, and an n-type semiconductorfilm 803 that are stacked in this order.

The conductive film 811, which serves as the wiring 111 (PR), iselectrically connected to the p-type semiconductor film 801 thatfunctions as one of the electrodes (the anode) of the photoelectricconversion element 100.

A conductive film 841 functions as a wiring for connecting one of thesource and the drain of the transistor 120 to the conductive film 813.

A conductive film 842 functions as one of the source and the drain ofthe first switching element 121.

A conductive film 843 functions as a wiring for connecting one of thesource and the drain of the first switching element 121 in the firstsignal output circuit 101 to one of the source and the drain of thefirst switching element 121 in the second signal output circuit 102.

A conductive film 844 functions as one of the source and the drain ofthe transistor 120.

A conductive film 845 functions as the other of the source and the drainof the first switching element 121.

A conductive film 846 functions as a wiring for connecting the other ofthe source and the drain of the transistor 120 to one of the source andthe drain of the second switching element 122.

A conductive film 847 functions as the gate of the second switchingelement 122 in the first signal output circuit 101.

A conductive film 848 functions as the gate of the second switchingelement 122 in the second signal output circuit 102.

A conductive film 849 functions as a wiring for connecting the gate ofthe second switching element 122 in the first signal output circuit 101to the conductive film 816.

A conductive film 850 functions as a wiring for connecting the gate ofthe second switching element 122 in the second signal output circuit tothe conductive film 836.

The conductive films 812, 814, 816, 832, 834, 836, 841, 843, 847, and848 can be formed by processing one conductive film formed over aninsulating surface into a desired shape. Over these conductive films, agate insulating film 862 is formed. Further, the conductive films 811,813, 815, 842, 844, 845, 846, 849, and 850 can be formed by processingone conductive film formed over the gate insulating film 862 into adesired shape.

Over the conductive films 811, 813, 815, 842, 844, 845, 846, 849, and850, an insulating film 863 and an insulating film 864 are formed, andover the insulating films 863 and 864, a conductive film 870 is formed.

It is preferable to use an oxide semiconductor for a semiconductor layer880 of the first switching element 121. In order to hold chargegenerated by light entering the photoelectric conversion element 100 inthe conductive film 812 (FD1) (or the conductive film 832 (FD2)) for along time, a transistor having extremely low off-state current ispreferably used as the first switching element 121 electricallyconnected to the conductive film. For that reason, the use of an oxidesemiconductor material for the semiconductor layer 880 can increase theperformance of the photodetector circuit.

In the photodetector circuit in FIGS. 8A and 8B, the elements such asthe transistors and the photoelectric conversion element 100 may overlapeach other. This configuration can increase the pixel density and thuscan increase the resolution of an imaging device. In addition, the areaof the photoelectric conversion element 100 can be increased, and thesensitivity of the imaging device can be increased as a result.

<Example of Layout of Photodetector Circuit in FIG. 3A>

FIG. 9A is a top view of the photodetector circuit illustrated in FIG.3A, and FIG. 9B is a cross-sectional view along the dashed-dotted lineB1-B2 in FIG. 9A.

The photodetector circuit includes, over a substrate 960 on which aninsulating film 961 is formed, a conductive film 911 serving as thewiring 111 (PR), a conductive film 912 serving as the wiring 112 (FD1)in the first signal output circuit 301, a conductive film 932 serving asthe wiring 132 (FD2) in the second signal output circuit 302, aconductive film 913 serving as the wiring 113 (VR), a conductive film914 serving as the wiring 114 (TX1) in the first signal output circuit301, a conductive film 934 serving as the wiring 134 (TX2) in the secondsignal output circuit 302, a conductive film 915 serving as the wiring315 (OUT1) in the first signal output circuit 301, a conductive film 935serving as the wiring 335 (OUT2) in the second signal output circuit302, and a conductive film 916 serving as the wiring 316 (SE).

The photoelectric conversion element 100 includes a p-type semiconductorfilm 901, an i-type semiconductor film 902, and an n-type semiconductorfilm 903 that are stacked in this order.

The conductive film 911, which serves as the wiring 111 (PR), iselectrically connected to the p-type semiconductor film 901 thatfunctions as one of the electrodes (the anode) of the photoelectricconversion element 100.

A conductive film 941 is connected to the conductive film 913 serving asthe wiring 113 (VR) and functions as part of the wiring 113 (VR).

A conductive film 942 is connected to the conductive film 914 serving asthe wiring 114 (TX1) or the conductive film 934 serving as the wiring134 (TX2), and functions as the gate of the first switching element 121.

A conductive film 943 functions as one of the source and the drain ofthe first switching element 121.

A conductive film 944 functions as the other of the source and the drainof the first switching element 121.

A conductive film 945 functions as the other of the source and the drainof the transistor 120 and one of the source and the drain of the secondswitching element 122.

A conductive film 946 functions as a wiring for connecting theconductive film 911 to the p-type semiconductor film 901.

The conductive films 911, 912, 916, 932, 941, and 942 can be formed byprocessing one conductive film formed over an insulating surface into adesired shape. Over these conductive films, a gate insulating film 962is formed. Further, the conductive films 913, 914, 915, 934, 935, 943,944, 945, and 946 can be formed by processing one conductive film formedover the gate insulating film 962 into a desired shape.

Further, over the conductive films 911, 912, 916, 932, 941, and 942, aninsulating film 963 and an insulating film 964 are formed, and over theinsulating films 963 and 964, a conductive film 970 is formed.

It is preferable to use an oxide semiconductor for a semiconductor layer980 of the first switching element 121. In order to hold chargegenerated by light entering the photoelectric conversion element 100 inthe conductive film 912 (FD1) (or the conductive film 932 (FD2)) for along time, a transistor having extremely low off-state current ispreferably used as the first switching element 121 electricallyconnected to the conductive film. For that reason, the use of an oxidesemiconductor material for the semiconductor layer 980 can increase theperformance of the photodetector circuit.

In the photodetector circuit in FIGS. 8A and 8B, the elements such asthe transistors and the photoelectric conversion element 100 may overlapeach other. This configuration can increase the pixel density and thuscan increase the resolution of an imaging device. In addition, the areaof the photoelectric conversion element 100 can be increased, and thesensitivity of the imaging device can be increased as a result.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 5

The photodetector circuit described in any of the above embodiments canbe provided in a variety of semiconductor devices. In this embodiment,as an example of a semiconductor device including a photodetectorcircuit, a radiation imaging device in which adverse effect of afterglowis reduced by including the photodetector circuit described in any ofthe above embodiments will be described with reference to FIGS. 10A and10B and FIGS. 11A to 11D.

Further, an image display device having a touch panel function obtainedby including the photodetector circuit described in any of the aboveembodiments will be described with reference to FIG. 12, FIG. 13, andFIGS. 14A and 14B.

<Structure Example of Radiation Imaging Device>

A structure of a radiation imaging device including the photodetectorcircuit described in any of the above embodiments will be described withreference to FIGS. 10A and 10B and FIGS. 1A to 11D.

As illustrated in FIG. 10A, a radiation imaging device 1000 includes aradiation emission portion 1001, a scintillator 1004 which receivesradiation 1002 output from the radiation emission portion 1001 andoutputs light 1003, a photodetector mechanism 1005 which outputs adetection signal corresponding to the amount of incident light 1003, andan image signal generation portion 1006 which generates an image signalby using a detection signal output from the photodetector mechanism1005. Further, the radiation imaging device 1000 is connected to animage display device 1007 and the image display device 1007 receives animage signal output from the image signal generation portion 1006, sothat internal data and the like of an object 1008 is displayed on theimage display device 1007.

A structure example of the photodetector mechanism 1005 will bedescribed below with reference to FIG. 10B.

<Structure Example of Photodetector Mechanism>

The photodetector mechanism 1005 described in this embodiment includes aphotodetector portion 1010 in which photodetector circuits 1012 arearranged in a matrix of m rows and n columns, and a photodetectorcircuit control portion 1020 including a first photodetector circuitdriver 1021 and a second photodetector circuit driver 1022 forcontrolling the photodetector circuits 1012.

As the photodetector circuit 1012, the photodetector circuit illustratedin FIG. 1A is used (needless to say, the photodetector circuit 1012 isnot limited thereto).

The first photodetector circuit driver 1021 has a function of generatinga signal output to the wiring 113 (VR) and the wiring 111 (PR) and afunction of extracting detection signals, which are output from thefirst signal output circuit 101 and the second signal output circuit102, from the wiring 115 (OUT) in a selected row. Note that the firstphotodetector circuit driver 1021 is connected to the image signalgeneration portion 1006 which generates an image signal that is lessaffected by afterglow.

In addition, the first photodetector circuit driver 1021 includes aprecharge circuit, and has a function of setting the potential of thewiring 115 (OUT) to a predetermined potential. Note that the firstphotodetector circuit driver 1021 can have a structure in which outputof an analog signal from a photodetector circuit is extracted as it isto the outside of the radiation imaging device 1000 with the use of anoperational amplifier or the like or a structure in which an analogsignal is converted into a digital signal with the use of an A/Dconverter circuit and extracted to the outside of the radiation imagingdevice 1000.

The second photodetector circuit driver 1022 has a function ofgenerating a signal output to the wiring 114 (TX1), the wiring 134(TX2), the wiring 116 (SE1), and the wiring 136 (SE2).

The above is the description of the structure example of thephotodetector mechanism 1005.

<Operation Example of Radiation Imaging Device>

Next, an example of the operation of the radiation imaging device 1000having the above-described structure will be described with reference toFIGS. 11A to 11D.

In the case where moving images, for example, for monitoring blood flowin vessels or temporally continuous still images are taken with aradiation imaging device, it is necessary to increase the timeresolution of the radiation imaging device to obtain high-definitionimages; thus, it is desired that a period after stop of radiationemission before start of the next radiation emission be as short aspossible.

However, in the case where the period after stop of radiation emissionbefore start of the next radiation emission is short, light due toafterglow is output from a scintillator at the start of the nextradiation emission in some cases.

When radiation emission starts in such a state, light output from thescintillator is a combination of light obtained by radiation emissionand light due to afterglows of previous and earlier radiation emission;thus, a difference may arise between the amount of radiation through theobject 1008, which is received by the scintillator 1004, and datacorresponding to the amount of radiation received by the photodetectormechanism 1005, which is output from the photodetector mechanism 1005.

In view of the above, first, as illustrated in FIGS. 11A and 11B, light1101 output from the scintillator 1004 in a period 1111 just before thestart of the next radiation emission is received by the photoelectricconversion element 100 in each of the photodetector circuits 1012 andpotentials (data) (hereinafter, also referred to as potentials A) basedon the amount of incident light are held in the first signal outputcircuits 101.

The light 1101 output from the scintillator 1004 in the period 1111 canbe regarded as light due to afterglows of the previous and earlierradiation emission.

Next, as illustrated in FIGS. 11C and 11D, light 1102 output from thescintillator 1004 in a period 1112 during which the next radiationemission is performed is received by the photoelectric conversionelement 100 in each of the photodetector circuits 1012 and potentials(data) (hereinafter, also referred to as potentials B) based on theamount of incident light are held in the second signal output circuits102.

The light 1102 output from the scintillator 1004 in the period 1112 canbe regarded as light due to afterglows of the previous and earlierradiation emission.

Next, after the potentials A and the potentials B are held in all thephotodetector circuits 1012, a detection signal including the potentialA as data and a detection signal including the potential B as data areoutput from each of the photodetector circuits 1012 to the image signalgeneration portion 1006.

Then, in the image signal generation portion 1006, an image signal (forone pixel) is generated using a difference between the two detectionsignals input from each of the photodetector circuits 1012 and imagingdata is displayed on the image display device 1007 with the use of theimage signal.

Here, the photodetector circuit 1012 has a structure in which onephotoelectric conversion element and one signal output circuit areprovided.

In the structure, when data held in the signal output circuit remains inthe period 1111, a potential (data) in the period 1112 cannot beobtained accurately. In other words, since a potential (data) in theperiod 1112 is added to a potential (data) in the period 1111, output ofa detection signal corresponding to the potential (data) (an outputoperation) and reset of the potential (data) held in the signal outputcircuit (an reset operation) are necessarily performed before the startof the period 1112.

Since the amount of light emitted by afterglow is decreased as timepasses, as an interval from the end of the period 1111 to the start ofthe period 1112 increases, an accurate image signal is less likely to beobtained when an image signal is generated using a difference betweentwo detection signals as described above. Particularly in the case wherethe amount of temporal change of afterglow is large, the above problembecomes significant.

In contrast, in the case where in the photodetector circuit 1012, thetwo signal output circuits (the first signal output circuit 101 and thesecond signal output circuit 102) are connected to the photoelectricconversion element 100 as illustrated in FIG. 1A, turning off the firstswitching element 121 in the first signal output circuit 101 enables apotential (data) in the period 1111 to be held in the first signaloutput circuit 101 and further, only performing a reset operation on thesecond signal output circuit 102 after the period 1111 enables apotential (data) in the period 1112 to start to be obtained using thephotoelectric conversion element 100 and the second signal outputcircuit 102. Note that in the case where the photodetector circuitillustrated in FIG. 3A is used, a reset operation performed after theperiod 1111 is not necessary in some cases.

Thus, an image signal generated using a difference between two detectionsignals input from each of the photodetector circuits 1012 is anaccurate image signal that is less affected by afterglow.

The above is the description of the radiation imaging device includingthe photodetector circuit described in any of the above embodiments.

Although the image signal generation portion 1006 is provided in theradiation imaging device 1000 so as to be connected to the photodetectormechanism 1005 in FIGS. 10A and 10B, the image signal generation portion1006 may be provided in the photodetector mechanism 1005. Alternatively,the image signal generation portion 1006 may be provided outside theradiation imaging device 1000.

Further, although the image display device 1007 is provided outside theradiation imaging device 1000 in FIGS. 10A and 10B, the image displaydevice 1007 may be provided in the radiation imaging device 1000.

<Structure Example of Image Display Device>

FIG. 12 illustrates an example of a structure of an image display deviceincluding a plurality of pixels and a driver for driving the pluralityof pixels.

An image display device 1200 includes a display portion 1240, a displayelement control portion 1220, and a photodetector circuit controlportion 1230. The display portion 1240 includes a plurality of pixels1210 arranged in a matrix.

FIG. 12 illustrates an example where the pixel 1210 includes one displayelement 1201R emitting red light, one display element 1201G emittinggreen light, one display element 1201B emitting blue light, and onephotodetector circuit 1202. The structure of the photodetector circuit1202 can be similar to that described in any of the above embodimentscan be used.

An example of a configuration of the pixel 1210 is described below withreference to FIG. 13.

<Configuration Example of Pixel>

The pixel 1210 described in this embodiment includes three displayelements (the display element 1201R, the display element 1201G, and thedisplay element 1201B) and one photodetector circuit 1202. Using thepixel 1210 as a basic configuration, a plurality of pixels 1210 arearranged in a matrix of m rows and n columns and form a display screenthat also serves as a data input region. FIG. 13 illustrates an exampleof the case where the photodetector circuit having the configuration inFIG. 1A is used as the photodetector circuit 1202 in the pixel 1210.

Note that the number of display elements and the number of photodetectorcircuits included in each pixel is not limited to those illustrated inFIG. 13. The density of the photodetector circuits and that of thedisplay elements may be the same or different. That is, onephotodetector circuit may be provided for one display element; onephotodetector circuit may be provided for two or more display elements;or one display element may be provided for two or more photodetectorcircuits.

FIG. 13 illustrates a configuration where the display element 1201R, thedisplay element 1201G, and the display element 1201B each include aliquid crystal element 1250 is illustrated as an example. The displayelement 1201R, the display element 1201G, and the display element 1201Beach include the liquid crystal element 1250, a transistor 1252 servingas a switching element for controlling the operation of the liquidcrystal element 1250, and a capacitor 1254. The liquid crystal element1250 includes a pixel electrode, a counter electrode, and a liquidcrystal layer to which a voltage is applied by the pixel electrode andthe counter electrode.

Although not illustrated, a red color filter, a green color filter, anda blue color filter are provided on the light extraction side of theliquid crystal element 1250 in the display element 1201R, the liquidcrystal element 1250 in the display element 1201G, and the liquidcrystal element 1250 in the display element 1201B, respectively.

A gate of the transistor 1252 is connected to a scan line GL (GL1 orGL2). One of a source and a drain of the transistor 1252 is connected toa signal line SL (SL1 or SL2), and the other of the source and the drainof the transistor 1252 is connected to a pixel electrode of the liquidcrystal element 1250. One of a pair of electrodes of the capacitor 1254is connected to the pixel electrode of the liquid crystal element 1250,and the other of the pair of electrodes of the capacitor 1254 isconnected to a wiring COM supplied with a fixed potential. The signalline SL is supplied with a potential corresponding to an image to bedisplayed. When the transistor 1252 is turned on with a signal of thescan line GL, the potential of the signal line SL is supplied to one ofthe pair of the electrodes of the capacitor 1254 and the pixel electrodeof the liquid crystal element 1250. The capacitor 1254 holds chargecorresponding to voltage applied to the liquid crystal layer. Contrast(gray scale) of light passing through the liquid crystal layer is madeby utilizing the change in the polarization direction of the liquidcrystal layer with voltage application, and images are displayed. Aslight passing through the liquid crystal layer, light emitted from thebacklight is used.

In the configuration in FIG. 13, the operation of the display elementsarranged in a matrix can be similar to that in a known display device.

Note that as the transistor 1252, the transistor including an oxidesemiconductor material in a channel formation region, which is describedin any of the above embodiments, can be used. In the case of using thetransistor, since its off-state current is extremely low, the capacitor1254 is not necessarily provided.

Note that each of the display elements 1201R, 1201G, and 1201B mayfurther include another circuit element such as a transistor, a diode, aresistor, a capacitor, or an inductor as needed.

Although FIG. 13 illustrates the case where the display element 1201R,the display element 1201G, and the display element 1201B each includethe liquid crystal element 1250, another element such as alight-emitting element may be included. The light-emitting element is anelement whose luminance is controlled with current or voltage, andspecific examples thereof are a light emitting diode and an organiclight-emitting diode (OLED).

The above is the description of the configuration example of the pixel1210.

The display element control portion 1220 includes a first displayelement driver 1221 which has a function of controlling the displayelements 1201 and inputs a signal to the display element 1201 through asignal line through which an image signal is transmitted (also referredto as a “source signal line”) and a second display element driver 1222which inputs a signal to the display element 1201 through a scan line(also referred to as a “gate signal line”). For example, the firstdisplay element driver 1221 has a function of giving a predeterminedpotential to the display elements 1201 in the pixels 1210 placed in theselected line. Further, the second display element driver 1222 has afunction of selecting the display elements 1201 included in the pixelsplaced in a particular row.

The photodetector circuit control portion 1230 includes drivers forcontrolling the photodetector circuits 1202, and specifically a firstphotodetector circuit driver 1231 which faces the first display elementdriver 1221 with the display portion 1240 provided therebetween, and asecond photodetector circuit driver 1232 which faces the second displayelement driver 1222 with the display portion 1240 provided therebetween.

The first photodetector circuit driver 1231 has a function of generatinga signal output to the wiring 111 (PR) and the wiring 113 (VR) and afunction of extracting an output signal of a photodetector circuit inthe pixel 1210 in a selected row from the wiring 115 (OUT). Note thatthe first photodetector circuit driver 1231 is connected to a detectionsignal comparison portion 1260 which determines whether an object to bedetected exists over each pixel 1210 or not with the use of a pluralityof detection signals output from each pixel 1210.

In addition, the first photodetector circuit driver 1231 includes aprecharge circuit, and has a function of setting the potential of thewiring 115 (OUT) to a predetermined potential. Note that the firstphotodetector circuit driver 1231 can have a structure in which outputof an analog signal from a photodetector circuit is extracted as it isto the outside of the image display device 1200 with the use of anoperational amplifier or the like or a structure in which an analogsignal is converted into a digital signal with the use of an A/Dconverter circuit and extracted to the outside of the image displaydevice 1200.

The second photodetector circuit driver 1232 has a function ofgenerating a signal output to the wiring 114 (TX1), the wiring 134(TX2), the wiring 116 (SE1), and the wiring 136 (SE2).

The above is the description of the configuration example of the imagedisplay device 1200.

<Operation Example of Image Display Device>

Next, an example of the operation of the image display device having theabove-described configuration will be described with reference to FIGS.14A and 14B.

The photodetector circuit 1202 provided in the image display device 1200can hold potentials (data) including the amount of light entering thephotoelectric conversion element 100 in given periods as data inaccordance with the number of signal output circuits included in thephotodetector circuit 1202, as described in the above embodiments.

For example, in the photodetector circuit in FIG. 13, potentials (data)in two periods can be held by using the first signal output circuit 101and the second signal output circuit 102. Note that in this embodiment,the two periods are denoted by a period A and a period B, and the periodB comes after the period A.

The period A is a period during which an object to be detected, such asa finger, does not exist over the display element 1201 as illustrated inFIG. 14A. Light (image) output from the display element 1201 with theuse of the first display element driver 1221 and the second displayelement driver 1222 is output to the outside through a liquid crystallayer 1401, a pair of alignment films 1402 between which the liquidcrystal layer 1401 is sandwiched, a pair of electrodes 1403 betweenwhich the pair of alignment films 1402 are sandwiched, a color filter1404, a substrate 1405, and the like.

Thus, slightly reflected light which is reflected by the substrate 1405or the like, external light, or the like is input to the photodetectorcircuit 1202, and a potential (data) (hereinafter, also referred to as apotential C) including the amount of incident light in the period A asdata is held in the first signal output circuit 101.

The period B is a period during which an object 1410 to be detectedexists over the display element 1201 as illustrated in FIG. 14B, andlight (image) output from the display element 1201 with the use of thefirst display element driver 1221 and the second display element driver1222 is partly absorbed by the object 1410, the other light enters thephotodetector circuit 1202, and a potential (data) (hereinafter, alsoreferred to as a potential D) including the amount of incident light inthe period B as data is held in the second signal output circuit 102.

Note that the amount of light entering the photodetector circuit 1202 inthe period B is much larger than the amount of incident light in theperiod A.

After that, the potential C and the potential D are held in all thepixels 1210 in the display portion 1240, and then, a detection signalincluding the potential C as data and a detection signal including thepotential D as data are output from each of the pixels 1210 to thedetection signal comparison portion 1260.

Then, in the detection signal comparison portion 1260, two detectionsignals input from each of the pixels 1210 are compared. In the casewhere a difference of a predetermined value (which may be determined bya practitioner as appropriate) or more is found, it is judged that theobject 1410 exists over the pixel 1210.

Here, the case where the photodetector circuit 1202 includes onephotoelectric conversion element and one signal output circuit isdescribed.

In the structure, when data held in the signal output circuit remains inthe period A, a potential (data) in the period B cannot be obtainedaccurately. In other words, since a potential (data) in the period B isadded to a potential (data) in the period A, output of a detectionsignal corresponding to a potential (data) (output operation) and resetof a potential (data) held in the signal output circuit (resetoperation) are necessarily performed by the start of the period B.

Therefore, after the end of the period A and before the start of theperiod B, for example, in the case where an output operation isperformed on data obtained in the period A and the object 1410 passesover the display element 1201 in a period during which a reset operationis performed on the data obtained in the period A, whether the object1410 exists over each pixel 1210 or not cannot sometimes be judgedaccurately by the detection signal comparison portion 1260, even usingdata obtained in the period A and data obtained in the period B.

In contrast, in the image display device 1200 described in thisembodiment, two signal output circuits (the first signal output circuit101 and the second signal output circuit 102) are connected to thephotoelectric conversion element 100 as illustrated in FIG. 13; thus,turning off the first switching element 121 in the first signal outputcircuit 101 enables a potential (data) in the period A to be held in thefirst signal output circuit. Further, only performing a reset operationon the second signal output circuit 102 after the period A enables apotential (data) in the period B to start to be obtained using thephotoelectric conversion element 100 and the second signal outputcircuit 102.

Thus, even in the case where the object 1410 moves extremely quickly,whether the object 1410 exists over each pixel 1210 or not can be judgedaccurately.

The above is the description of the image display device including thephotodetector circuit described in any of the above embodiments.

Although the detection signal comparison portion 1260 is provided in theimage display device 1200 so as to be connected to the firstphotodetector circuit driver 1231 in FIG. 12, the detection signalcomparison portion 1260 may be provided in the first photodetectorcircuit driver 1231. Alternatively, the detection signal comparisonportion 1260 may be provided outside the image display device 1200.

This application is based on Japanese Patent Application serial no.2012-200495 filed with Japan Patent Office on Sep. 12, 2012, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A photodetector circuit comprising: a photoelectricconversion element; a first transistor; and a second transistor, whereinthe first transistor is configured to hold a first potential of a firstnode in accordance with an amount of light entering the photoelectricconversion element when light is emitted from a light source, andwherein the second transistor is configured to hold a second potentialof a second node in accordance with an amount of light entering thephotoelectric conversion element when light is not emitted from thelight source.
 3. The photodetector circuit according to claim 2, whereinthe first transistor comprises a channel formation region comprising anoxide semiconductor, and wherein the second transistor comprises achannel formation region comprising an oxide semiconductor.
 4. Thephotodetector circuit according to claim 3, wherein the photoelectricconversion element is configured to generate first charge correspondingto the first potential during a first potential generation period wherelight is emitted from the light source to the photoelectric conversionelement, wherein the photoelectric conversion element is configured togenerate second charge corresponding to the second potential during asecond potential generation period where light is not emitted from thelight source after the first potential generation period, wherein thephotodetector circuit is configured to output a signal corresponding tothe first potential during a first output period after the secondpotential generation period, and wherein the photodetector circuit isconfigured to output a signal corresponding to the second potentialduring a second output period after the first output period.
 5. Thephotodetector circuit according to claim 4, wherein the first transistoris configured to hold the first potential from the first potentialgeneration period to the first output period, and wherein the secondtransistor is configured to hold the second potential from the secondpotential generation period to the second output period.
 6. Thephotodetector circuit according to claim 3, wherein the photoelectricconversion element is configured to generate first charge correspondingto the first potential during a first potential generation period wherelight is emitted from the light source to the photoelectric conversionelement, wherein the photoelectric conversion element is configured togenerate second charge corresponding to the second potential during asecond potential generation period where light is not emitted from thelight source after the first potential generation period, wherein thephotodetector circuit is configured to output a signal corresponding tothe first potential during an output period after the second potentialgeneration period, and wherein the photodetector circuit is configuredto output a signal corresponding to the second potential during theoutput period.
 7. The photodetector circuit according to claim 6,wherein the first transistor is configured to hold the first potentialfrom the first potential generation period to the output period, andwherein the second transistor is configured to hold the second potentialfrom the second potential generation period to the output period.
 8. Animaging device comprising: a scintillator; and a photodetector circuitcomprising: a photoelectric conversion element; a first transistor; anda second transistor, wherein the first transistor is configured to holda first potential of a first node in accordance with an amount of lightentering the photoelectric conversion element from the scintillator whenradiation is emitted from a radiation source, and wherein the secondtransistor is configured to hold a second potential of a second node inaccordance with an amount of light entering the photoelectric conversionelement from the scintillator when radiation is not emitted from theradiation source.
 9. The imaging device according to claim 8, whereinthe first transistor comprises a channel formation region comprising anoxide semiconductor, and wherein the second transistor comprises achannel formation region comprising an oxide semiconductor.
 10. Theimaging device according to claim 9, wherein the photoelectricconversion element is configured to generate first charge correspondingto the first potential during a first potential generation period whereradiation is emitted from the radiation source to the scintillator,wherein the photoelectric conversion element is configured to generatesecond charge corresponding to the second potential during a secondpotential generation period where radiation is not emitted from theradiation source after the first potential generation period, whereinthe photodetector circuit is configured to output a signal correspondingto the first potential during a first output period after the secondpotential generation period, and wherein the photodetector circuit isconfigured to output a signal corresponding to the second potentialduring a second output period after the first output period.
 11. Theimaging device according to claim 10, wherein the first transistor isconfigured to hold the first potential from the first potentialgeneration period to the first output period, and wherein the secondtransistor is configured to hold the second potential from the secondpotential generation period to the second output period.
 12. The imagingdevice according to claim 9, wherein the photoelectric conversionelement is configured to generate first charge corresponding to thefirst potential during a first potential generation period whereradiation is emitted from the radiation source to the scintillator,wherein the photoelectric conversion element is configured to generatesecond charge corresponding to the second potential during a secondpotential generation period where radiation is not emitted from theradiation source after the first potential generation period, whereinthe photodetector circuit is configured to output a signal correspondingto the first potential during an output period after the secondpotential generation period, and wherein the photodetector circuit isconfigured to output a signal corresponding to the second potentialduring the output period.
 13. The imaging device according to claim 12,wherein the first transistor is configured to hold the first potentialfrom the first potential generation period to the output period, andwherein the second transistor is configured to hold the second potentialfrom the second potential generation period to the output period.
 14. Animaging device comprising: a radiation source; a scintillator; and aphotodetector circuit comprising: a photoelectric conversion element; afirst transistor; and a second transistor, wherein the first transistoris configured to hold a first potential of a first node in accordancewith an amount of light entering the photoelectric conversion elementfrom the scintillator when radiation is emitted from the radiationsource, and wherein the second transistor is configured to hold a secondpotential of a second node in accordance with an amount of lightentering the photoelectric conversion element from the scintillator whenradiation is not emitted from the radiation source.
 15. The imagingdevice according to claim 14, wherein the first transistor comprises achannel formation region comprising an oxide semiconductor, and whereinthe second transistor comprises a channel formation region comprising anoxide semiconductor.
 16. The imaging device according to claim 15,wherein the photoelectric conversion element is configured to generatefirst charge corresponding to the first potential during a firstpotential generation period where radiation is emitted from theradiation source to the scintillator, wherein the photoelectricconversion element is configured to generate second charge correspondingto the second potential during a second potential generation periodwhere radiation is not emitted from the radiation source after the firstpotential generation period, wherein the photodetector circuit isconfigured to output a signal corresponding to the first potentialduring a first output period after the second potential generationperiod, and wherein the photodetector circuit is configured to output asignal corresponding to the second potential during a second outputperiod after the first output period.
 17. The imaging device accordingto claim 16, wherein the first transistor is configured to hold thefirst potential from the first potential generation period to the firstoutput period, and wherein the second transistor is configured to holdthe second potential from the second potential generation period to thesecond output period.
 18. The imaging device according to claim 15,wherein the photoelectric conversion element is configured to generatefirst charge corresponding to the first potential during a firstpotential generation period where radiation is emitted from theradiation source to the scintillator, wherein the photoelectricconversion element is configured to generate second charge correspondingto the second potential during a second potential generation periodwhere radiation is not emitted from the radiation source after the firstpotential generation period, wherein the photodetector circuit isconfigured to output a signal corresponding to the first potentialduring an output period after the second potential generation period,and wherein the photodetector circuit is configured to output a signalcorresponding to the second potential during the output period.
 19. Theimaging device according to claim 18, wherein the first transistor isconfigured to hold the first potential from the first potentialgeneration period to the output period, and wherein the secondtransistor is configured to hold the second potential from the secondpotential generation period to the output period